feat(flags): track feature flag evaluations in span attributes

[aliu39]

Based off https://github.com/getsentry/sentry-python/pull/4280/files

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[aliu39]

304 lines after changes

[aliu39]

Since users can call setAttribute, can be user defined I thought it best to prefix with sentry. See SpanAttributes defn

[billyvg]

Should this be a buffer so that we can evict the least recently added flags (similar to what we do w/ breadcrumbs)? Silently ignoring the flag here feels not great.

[aliu39]

It's what we do on in python atm, cc @cmanallen. We could have a buffer but need to make sure the attribute is serializable - Doesn't look like SpanAttributeValue allows nested objects. Maybe we could store a buffer of keys internally

[cmanallen]

For spans the oldest flag evaluations are the most important because they're likely to have the largest performance impact. We drop late arrivals due to request size concerns.

[AbhiPrasad]

Is it expected that this will be used by users? Like they will call span.addFeatureFlag?

A method on the span break our compat with OTEL spans, which we use for duck typing in some scenarios. I would prefer if we made this a helper method we export from @sentry/core instead. You can see an example of this with setHttpStatus:

sentry-javascript/packages/core/src/tracing/spanstatus.ts

Line 61 in dcdf074

export function setHttpStatus(span: Span, httpStatus: number): void {

We're basically just extracting addFlagToActiveSpan into core, with a implementation that looks something like so:

export function addFlagToSpan(span: Span, name: string, value: unknown): void {
  if (!span || typeof value !== 'boolean') {
    return;
  }
  span.setAttribute(`${FEATURE_FLAG_ATTRIBUTE_PREFIX}${name}`, value);
}

[AbhiPrasad]

Adding a new method onto a span breaks our compatibility with OTEL, which we leverage for duck typing.

[aliu39]

This strategy makes sense, but how can we track the num current flags? (currently a protected field on the span)

[aliu39]

Is it expected that this will be used by users? Like they will call span.addFeatureFlag?

I'm thinking we can expose this as an API for users, following the pattern we have for adding flags to scope in featureFlagIntegration. It'd be the same as the addFeatureFlag method, without the need for an integration

[aliu39]

Opening a new PR w/revised implementation